The art has provided many ways for a direction finder (DF) to determine the direction to an RF source (target), mainly by various wave analysis procedures.
Direction finding techniques can be categorized in groups, those which find the direction of the target based on the received signal amplitude, based on the received signal phase, based on received signal timing, or those which are based on several of said attributes of the received signal.
One of the major challenges all direction-finding techniques face, in most situations, but mainly within a reflective environment, is to overcome the multipath reflections problem. Multipath reflections can cause false indications regarding the direction of the targeted RF source. Reflection of waves is expected from nearby objects, such as walls, or metallic objects. Waves transmitted from a target may be scattered and reflected from nearby objects such as wall, and arrive to the direction finder via many waves and from many directions. The reflected waves are weaker due to the following facts: (a) the reflected waves travel a loner path; (b) The reflected waves are scattered to many directions; and (c) the reflected waves from an object suffer from reflection losses. The reflected waves arrive at the DF later than the direct wave due to the longer path. These reflections are combined with the direct wave, distorting the amplitude, phase, and time of arrival of the signal. In prior art direction finding techniques that are based on measuring the signals amplitude, phase, or time of arrival, these multi-path reflections cause sever errors in the direction finding.
Amplitude-based direction finding techniques: These direction finding techniques use one or more antennas. An example of a single antenna direction finding is a rotational directional antenna. The direction from which the received signal strength (RSS) or received signal strength indication (RSSI), or equivalent thereof is the highest, is the expected direction to the target. Amplitude based directional finders that use several antennas measure the RSS/RSSI at each antenna and calculate from these amplitude differences the Angle of Arrival (AOA) of the signal. An example for an amplitude directional finder which uses several antennas is the monopulse system.
Additional techniques assess the distance to the target, based on the signal strength, and by triangulating several measurements calculate the location or the direction to the target.
Phase-based direction finding techniques: These directional finders use two or more antennas and measure the phase difference of the arrival of a signal in plurality of antennas and calculate from these phase differences the AOA of the signal. This group includes, for example, interferometer direction finder, correlative interferometer direction finder, passed array systems, etc.
Time-based directional finder techniques: These directional finders are also known as TOA (Time of Arrival) type directional finders. They use two or more antennas and measure the time difference of the arrival of a signal to plurality of antennas and calculate from these differences the AOA of the signal. This group includes, for example short and long base TOA, DTOA (Differential Time of Arrival) etc.
Monopulse DF techniques: This technique is mainly used in ELINT (Electronic Intelligence) systems and radars, to find the direction from which a pulsed radar signal or echo is received. The signal is received in two or more directional antennas. The signals in the antennas, usually highly directional antennas, are added in phase to create a sum (Σ) signal, and added in opposite phase to create a Difference (Δ) signal, in one or two dimensions, azimuth, elevation or both. Based on the Σ and Δ signal strengths, the direction of the target is determined.
All said prior art techniques rely on one or more properties of the received signal, and therefore require relatively complicated calculations and analysis, and are also relatively expensive. Therefore, said techniques are generally not suitable for small size and relatively simple wireless personal devices, such as cellular phones, PDAs, digital cameras, remote-control devices. Such devices are small in size, are provided in many cases with two or more simple omni directional or very low gain directional antennas, and are relatively of low cost. Furthermore, in many cases such devices comprise of only one receiving channel for each antenna, and therefore are not suitable for using the abovementioned prior art techniques, unless significantly increasing their size, and or price.
US 2007/0293150 A1 discloses a compact antenna system for polarization sensitive null steering and direction finding.
EP1610258A1 discloses a tag reader/writer for communicating radio frequency identification tags through radio wave that includes position calculating section which calculates position of RFID tag based on estimated incoming direction of radio wave from RFID tag. The incoming direction of the radio wave is estimated by a direction estimation section, as the radio wave travels from the RFID tag to at least one antenna. However, this tag reader/writer also uses signal strength and is therefore limited to RSSI protocols.
The article “A new method to find the direction of radar signal” (by Li Jinrui et. al., Radar Proceedings, CIE International Conference of Volume, 8-10 Oct. 1996 pp:601-604) discloses a direction finding technique according to which a directional pulse amplitude information is obtained by taking an omni-directional channel as a reference and finding the pulse amplitude among directional beams. However, this technique (intended to look for radar pulses) uses signal strength and therefore requires many directional antennas and is limited to RSSI protocols.
It is an object of the present invention to provide such a direction finding technique and device that are invulnerable to reflections of the signal from nearby objects, such as walls.
It is an object of the present invention to provide a direction finding technique and device, for determining those wireless communicating devices that are located within a predefined direction sector of interest.
It is still an object of the present invention to provide such direction finding technique and device that are simple and reliable.
It is still another object of the present invention to provide a direction finding technique and device that do not depend on attributes of the signal such as its amplitude, phase, or time of arrival.
It is another object of the present invention to provide such direction finding technique and device that are compact in size, and therefore well adapted to small and relatively cheap personal devices, such as cellular phone, PDAs, digital camera, remote controls, etc.
It is still another object of the present invention to provide such direction finding technique and device that can further discriminate between wireless communicating devices that are located at the front and those that are located at the back of the device.
Other objects and advantages of the invention will become apparent as the description proceeds